1. Field of the Invention
The present invention relates to an information storage medium such as an optical disk for recording or erasing information by way of a phase transformation of a recording layer upon radiation of an light beam such as a laser beam.
2. Description of the Prior Art
A phase-transformation type optical disk is a typical example of an optical disk which permits information erasure. When a portion of a recording layer of a phase transformation-type optical disk is irradiated with a laser beam, that portion undergoes a phase transformation between a crystalline phase and an amorphous phase, depending on the conditions under which the laser beam is radiated, whereby information is either recorded thereon or erased therefrom.
Materials which ca undergo such phase transformation include semiconductors, semiconductor compounds, and metal alloys (such as Te, Ge, TeGe, InSe, SbSe, and SbTe). Such materials can be selectively set in a crystalline state or an amorphous state. The complex indices of refraction of these materials which is given by N=n-ik differ considerably between one of these states and the other. The above two states can be alternately reversed by means of a heat treatment process employing a laser beam, thereby enabling information to be either recorded or erased (S.R. Ovshinsky, Metallurgical Transactions 2, 641, 1971).
Besides the method described above, there exists a system is which information is recorded and erased by means of reversible phase transformation between different crystalline phases by changing the conditions under which a laser beam is radiated (Japanese Patent Disclosure (Kokai) Nos. Sho61-134944). A typical example of such a material is an In-Sb alloy.
Upon radiation of a low-power laser beam having a relatively long pulse, the grains of an In-Sb alloy thin film are converted into small-crystal grains. These small-crystal grains can be quickly grown into relatively large crystal grains upon radiation of a high-power laser beam having a short pulse. The complex indices of refraction of each of these two crystal structures differ considerably. When information is read from a recording layer, upon radiation of a laser beam thereinto, the crystal states are discriminated on the basis of a difference between amounts of light reflected by surface portions of the disk.
All of the materials mentioned above whose phase can be changed between amorphous and crystalline phases, thereby to the recording or erasure of information, have a low crystallization rate, as a result of which the respective periods of time required for initialization and information erasure are undersirably long.
In an optical disk utilizing an In-Sb recording layer for alternately changing from one crystalline phase to another, an In.sub.50 Sb.sub.50 intermetallic compound can be advantageously used due to its high crystallization rate, which facilitates rapid initialization and information erasure. However, it is difficult to actually record information for the reason that Sb segregation (described later) does not occur. When the In content of the recording layer is higher than the Sb content, complete phase transformation cannot always be assured, with the result that recording of information is sometimes not possible. Another variation is to make the Sb content of the recording layer slightly higher than the In content as compared with In.sub.50 Sb.sub.50. When the recording layer is irradiated with a laser beam, it is converted to a mixture phase of InSb intermetallic compound crystal grains and Sb crystal grains. The size of the Sb crystal grains changes in accordance with the conditions under which the laser beam is radiated, and the information recording signal level can be maintained. However, since Sb has a low crystal growth rate, the recording layer initialization time and erase time are prolonged, thereby rendering high-speed operation impossible. More specifically, there is a tendency for initialization failure and incomplete erasure to occur. In the write mode, when the optical disk is rotated at high speed, crystal grains cannot grow sufficiently, thus resulting in unsatisfactory recording.